Active Object

About 3 min

Intent

The Active Object design pattern provides a safe and reliable way to implement asynchronous behavior in concurrent systems. It achieves this by encapsulating tasks within objects that have their own thread and message queue. This separation keeps the main thread responsive and avoids issues like direct thread manipulation or shared state access.

Explanation

Real-world example

Imagine a busy restaurant where customers place orders with waiters. Instead of the waiters going to the kitchen to prepare the food themselves, they write the orders on slips and hand them to a dispatcher. The dispatcher manages a pool of chefs who prepare the meals asynchronously. Once a chef is free, they pick up the next order from the queue, prepare the dish, and notify the waiter when it's ready for serving.
In this analogy, the waiters represent the client threads, the dispatcher represents the scheduler, and the chefs represent the method execution in separate threads. This setup allows the waiters to continue taking orders without being blocked by the food preparation process, much like the Active Object pattern decouples method invocation from execution to enhance concurrency.

In plain words

The Active Object pattern decouples method execution from method invocation to improve concurrency and responsiveness in multithreaded applications.

Wikipedia says

The active object design pattern decouples method execution from method invocation for objects that each reside in their own thread of control.[1] The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests.
The pattern consists of six elements:
A proxy, which provides an interface towards clients with publicly accessible methods.
An interface which defines the method request on an active object.
A list of pending requests from clients.
A scheduler, which decides which request to execute next.
The implementation of the active object method.
A callback or variable for the client to receive the result.

Programmatic Example

The Orcs are known for their wildness and untameable soul. It seems like they have their own thread of control based on previous behavior. To implement a creature that has its own thread of control mechanism and expose its API only and not the execution itself, we can use the Active Object pattern.

public abstract class ActiveCreature {
    private final Logger logger = LoggerFactory.getLogger(ActiveCreature.class.getName());

    private BlockingQueue<Runnable> requests;

    private String name;

    private Thread thread;

    public ActiveCreature(String name) {
        this.name = name;
        this.requests = new LinkedBlockingQueue<Runnable>();
        thread = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        requests.take().run();
                    } catch (InterruptedException e) {
                        logger.error(e.getMessage());
                    }
                }
            }
        }
        );
        thread.start();
    }

    public void eat() throws InterruptedException {
        requests.put(new Runnable() {
                         @Override
                         public void run() {
                             logger.info("{} is eating!", name());
                             logger.info("{} has finished eating!", name());
                         }
                     }
        );
    }

    public void roam() throws InterruptedException {
        requests.put(new Runnable() {
                         @Override
                         public void run() {
                             logger.info("{} has started to roam the wastelands.", name());
                         }
                     }
        );
    }

    public String name() {
        return this.name;
    }
}

We can see that any class that will extend the ActiveCreature class will have its own thread of control to invoke and execute methods.

For example, the Orc class:

public class Orc extends ActiveCreature {

    public Orc(String name) {
        super(name);
    }
}

Now, we can create multiple creatures such as orcs, tell them to eat and roam, and they will execute it on their own thread of control:

public class App implements Runnable {

    private static final Logger logger = LoggerFactory.getLogger(App.class.getName());

    private static final int NUM_CREATURES = 3;

    public static void main(String[] args) {
        var app = new App();
        app.run();
    }

    @Override
    public void run() {
        List<ActiveCreature> creatures = new ArrayList<>();
        try {
            for (int i = 0; i < NUM_CREATURES; i++) {
                creatures.add(new Orc(Orc.class.getSimpleName() + i));
                creatures.get(i).eat();
                creatures.get(i).roam();
            }
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            logger.error(e.getMessage());
            Thread.currentThread().interrupt();
        } finally {
            for (int i = 0; i < NUM_CREATURES; i++) {
                creatures.get(i).kill(0);
            }
        }
    }
}

Program output:

09:00:02.501 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 is eating!
09:00:02.501 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 is eating!
09:00:02.501 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 is eating!
09:00:02.504 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 has finished eating!
09:00:02.504 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 has finished eating!
09:00:02.504 [Thread-0] INFO com.iluwatar.activeobject.ActiveCreature -- Orc0 has started to roam in the wastelands.
09:00:02.504 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 has finished eating!
09:00:02.504 [Thread-1] INFO com.iluwatar.activeobject.ActiveCreature -- Orc1 has started to roam in the wastelands.
09:00:02.504 [Thread-2] INFO com.iluwatar.activeobject.ActiveCreature -- Orc2 has started to roam in the wastelands.

Applicability

When you need to perform long-running operations without blocking the main thread.
When you need to interact with external resources asynchronously.
When you want to improve the responsiveness of your application.
When you need to manage concurrent tasks in a modular and maintainable way.

Tutorials

Android and Java Concurrency: The Active Object Pattern(Douglas Schmidt)open in new window

Known Uses

Real-time trading systems where transaction requests are handled asynchronously.
GUIs where long-running tasks are executed in the background without freezing the user interface.
Game programming to handle concurrent updates to game state or AI computations.

Consequences

Benefits:

Improves responsiveness of the main thread.
Encapsulates concurrency concerns within objects.
Promotes better code organization and maintainability.
Provides thread safety and avoids shared state access problems.

Trade-offs:

Introduces additional overhead due to message passing and thread management.
May not be suitable for all types of concurrency problems.

Commandopen in new window: Encapsulates a request as an object, similarly to how the Active Object pattern encapsulates method calls.
Promiseopen in new window: Provides a means to retrieve the result of an asynchronous method call, often used in conjunction with Active Object.
Proxyopen in new window: The Active Object pattern can use a proxy to handle method invocations asynchronously.